The biochemistry of carbon radicals remains poorly understood despite its direct relevance to oxygen radical pathology and the unique value of carbon radicals as mechanistic probes. The work proposed here builds on key advances made during the expiring period of support, particularly (a) validation of a rationale for the fact that radicals are almost invariably produced by peroxidases but only under certain conditions by monooxygenases, (b) successful expression of catalytically active horseradish peroxidase in a baculovirus system, and (c) development of novel free radical probes. The first goal is to continue to elucidate the function of peroxidases, with initial emphasis on the structural features that determine the location of unpaired electrons in the protein, the mechanism of free radical generation, and the possible catalysis of monooxygenase reactions. The second goal is to extend our growing understanding of plant and fungal peroxidases to the physiologically relevant mammalian enzymes, particularly myeloperoxidase and thyroid peroxidase. The third goal is to construct new radical probes and to employ them to characterize biological radicals. The fourth goal is to explore the biological fates of carbon radicals, particularly their oxidation to cations and reactions with proteins. The collective intent of these studies is to advance our understanding of the biochemistry of carbon radicals, particularly the mechanisms by which they are formed and quenched, and to clarify their toxicological potential.